This invention relates to a method of treating waste water of a nuclear power plant and a system adapted to carry such method into practice.
In a nuclear power plant, waste water which is commonly referred to as floor drain is produced. For example, the waste water is produced when the drain dropping onto the floor from various machines and instruments of the nuclear power plant is washed away. Such waste water carries radioactively contaminated impurities. The waste water is also produced when steam seeping through a valve and condensed back into water drops onto the floor and the floor is cleaned by washing away the leak, when machines and instruments making up the nuclear power plant are cleaned before they are repaired, and when objects, as working clothes polluted with radioactivity or radioactive impurities are cleaned or laundered. It is not permissible to allow such waste water to flow out of the nuclear power plant without giving any treatment thereto. Thus the waste water is usually treated by waste water ion-exchange treating means so as to reduce the radioactive impurities carried thereby. The waste water is supplied to a condensation storage tank after the concentration of the radioactivity carried thereby is lowered to an average of 1 .times. 10.sup.-6 .mu.ci/cc by passing it through the waste water ion-exchange treating means.
Meanwhile a portion of the water flowing through the main circulation system is introduced into the condensation storage tank immediately after the water has passed through a condensation desalinator. The main circulation system constitutes a steam-feed water cycle which connects a nuclear reactor, a turbine, a condenser, a feed water pump, and a feed water heater in the indicated order, the feed water heater being connected to the nuclear reactor.
Besides being used to cope with an accident in case of emergency, the water contained in the condensation storage tank is also used for operating the control rod drive apparatus during normal operation. The water used for operating the control rod drive apparatus is returned to the nuclear reactor. The water in the condensation storage tank is also used for providing sealing steam to the gland seal sections of the turbine. More specifically, the water in the condensation storage tank is supplied to a steam generator where it is converted into steam which is supplied to the gland seal sections of the turbine. The steam passing through the gland seal sections is condensed back into water in gland steam condenser, and the water produced by condensation is returned to the condenser through a condensation recovery tank. The volume of water flowing from the main circulation system into the condensation storage tank is substantially equal to the sum of the water used for operating the control rod drive apparatus and the volume of water supplied to the steam generator. Thus the water from the main circulation system accounts for the major portion of water introduced into the condensation storage tank. Because of the fact that the water passing through the main circulation system flows into the condensation storage tank as aforesaid, the concentration of the radioactivity carried by the water in the condensation storage tank is at a level of 1 .times. 10.sup.-4 .mu.ci/cc on an average.
Water which is substantially equal in volume to the waste water introduced into the condensation storage tank is withdrawn from the condensation storage tank and mixed in the sea water used in the condenser for cooling purposes. Thus the cleaning waste water is diluted and released into the sea. The water thus released into the sea carries radioactivity which has a concentration of 1 .times. 10.sup.-9 .mu.ci/cc. It has been proposed at the National Academy of Sciences in the United States of America that the allowable concentration of the radioactivity carried by the sea water returned from a nuclear power plant to the sea be less than 4 .times. 10.sup.-9 .mu.ci/cc. In recent years, 10 CFR 50 Appendix 1 which is a law of the United States of America has set the goal of reducing a dose of radiation from the radioactive gas to less than 1/100 of the prevailing value at the boundary of the site. As to the radioactive liquid, it is stipulated that a dose of radiation therefrom be less than 5 ci (except for tritium) per year.
Vigorous attempts have hitherto been made to demand that radioactive effluents from a nuclear power plant be reduced. It is likely that the demand of this nature would increase in intensity in the future as the people in general pay increasingly greater attention to nuclear power plants as a source of power supply.